Planning and preparation (PaP) are vital for high-quality teaching and thus for student learning. Nevertheless, the infrastructure to support teachers' PaP is often poorly developed. This project brings together the fields of subject didactic and school development in order to investigate PaP as part of an organised infrastructure and as qualified activity. We focus how infrastructure can be organised to support PaP in sustainable ways for professional development, efficient for transforming knowledge into relevant teaching activities, and is systematic and dialogic in its formative approach. The aim is to develop knowledge about a didactical collegial practice as well as about its role in the local school infrastructure. Our research-based hypothesis is that teachers PaP benefit from a collegial collaboration (Darling-Hammond et al., 2017) and that the implementation of such a designated work needs the local school management's active support (Jarl et al., 2017). We have followed the commencement and implementation of twelve subject-based planning teams (audio recorded meetings; documents). A modified version of the Tyler-model (Tyler, 1950), and tools from legitimation code theory (Maton, 2014) frames the analysis. The contribution to the fields of subject didactics bridge the knowledge gap about PaP qualities and competencies. It is likewise important for school improvement, as there is a parallel gap about leading PaP as a strategy for improvement. This knowledge is vital for improving school-reform programs, and teacher education.